What is Rainfall and How is it Created?

Rainfall is a type of precipitation that occurs when water vapor in the atmosphere condenses into droplets that can no longer be suspended in the air. The occurrence of rainfall is dependant upon several factors. Things such as prevailing wind directions, ground elevation, location within a continental mass, and location with respect to mountain ranges all have a major impact on the possibility of precipitation. For example, did you know that every island of Hawaii has a dry side and a wet side (meaning that one side of the island gets significantly more rainfall than the other)? This is because the mountains push the air mass upward (also known as Orographic Precipitation) which causes the water vapor to condense into droplets. Condensation, and thus rainfall, can occur anytime that an air mass containing water vapor is cooled.

Dynamic or Adiabatic Cooling – Natural cooling of the vapor cloud happens as it expands within the environment around it. This is probably the largest contributor of rainfall.

Temperature Differentials across Air Masses – When two masses of air collide, the warmer one will cool as heat is transferred to the cooler air mass. These air masses are also called fronts. Often, storms will occur at the interface between a cold and warm front.

Cooling by Radiation – Radiation is the tendency of warm systems to release heat energy into the cooler environment around it. This is similar to dynamic or adiabatic cooling except for the fact the this is not associated with thermal expansion.

Contact Cooling with the Earth – Often times when air physically contacts objects on the earth, it will be cooled. While this doesn’t account for too much rainfall, its effects can be seen as a morning dew or frost. Fog can also occur as a result of contact cooling.

Convective Storms – The sun heats the earth’s surface and the air adjacent to it. As the air warms, it expands and rises upward into the atmosphere (this is also known as an updraft). At a certain point, the air mass begins to experience dynamic cooling. This can often result in precipitation. Because the warm air rises and cools rapidly, a high intensity and relatively short duration storm will result. These types of storms are typical of the summer season do to the larger temperature differentials experienced at this time of the year.

Orographic Storms – This type of precipitation results when wind forces moist air landward towards mountainous terrain. The mountains ‘lift’ the moist air masses high into the atmosphere. Once the air rises, it cools and allows for precipitation to occur. The windward side of the mountain will receive much more rain than the leeward side (the side being protected from the wind).

Frontal Storms – Air masses will move from high pressure regions to low pressure regions to form cold & warm fronts. When the warm and cold fronts collide in the atmosphere, turbulence is created (which leads to thunder and lighting). The warm air is forced upward over the cold air because it is less dense. The frontal stratification and rapid lifting results in precipitation at interface of the two air masses.

Intensity - Intensity is a measure of how much rain has fallen during a specific amount of time. Usually this value will be reported in inches per hour or millimeters per hour. A very intense storm may drop rain at a rate of 5 to 10 inches per hour. while a tranquil sprinkle may precipitate at a rate of 0.2 to 0.4 inches per hour. Rainfall intensity is probably the single most important factor when engineers design highway and flood control structures.

Duration - Rainfall duration is the amount of time that it has been raining. Typically, longer duration storms have lower average intensities. High intensities are generally associated with short duration storms.

Frequency - Storm frequency is a measure of the probability or risk of occurrence. For a given storm duration, the probability that a rainfall event has of being equaled or exceeded in one year is known as its recurrence interval. The inverse of the recurrence interval is called the return period. Many of you have probably heard the "100-year" storm. A 100-year storm (return period) has a 1% chance (recurrence interval) of being equaled or exceeded in any given year. Also note that a 100-year storm does not always produce a 100-yr flood.

Temporal Distribution - Storms can have varying degrees of intensity for its duration. Some portions of the country are prone to early peaking storms while others experience the most intense moments near the middle or end. Temporal distribution has a large impact on the actual amount of runoff that is experienced on the ground.

Spatial distribution - The location and extent of storm can help determine the kind of rainfall event that will occur. The biggest impact that this rainfall characteristic has is on runoff volumes. For instance, a storm that occurs in the mountains may produce a different volume of runoff at a given location downstream then if that same storm were to be center above it. In engineering design, it is generally assumed that a storm is uniformly distributed above a drainage area.